Stagger element creation and rendering

These changes staggers the element creation and animations (also drops the animations after a configurable cutoff). It improves the painting performance mentioned in issue #24.

Here is the profile for the same scenario (as #24) with the updates. Painting time is cut by more than half. 

In our actual use case painting time (to stack view) is mere 64ms as Andy made other optimizations (like showing 3 items per stack and hiding all others etc).

![screen shot 2014-11-12 at 4 55 03 pm](https://cloud.githubusercontent.com/assets/8048953/5019322/b5db675c-6a8c-11e4-85f4-a7e995020a2e.png)